Metallurgists are aware that the addition of lithium reduces the density and increases the modulus of elasticity and mechanical strength of aluminum alloys. That explains the attraction to such alloys for uses in the aeronautical industry. However, it is known that such lithium-containing alloys often have unsatisfactory ductility and toughness.
Heretofore, aluminum-lithium alloys have been used only sparsely in aircraft structure. The relatively low use has been caused by casting difficulties associated with aluminum-lithium alloys and by their relatively low fracture toughness compared to other more conventional aluminum alloys. Aluminum-lithium alloys, however, provide a substantial lowering of density of aluminum alloys (as well as a relatively high strength to weight ratio), which has been found to be very important in decreasing the overall weight of structural materials. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge remains to obtain a good blend of fracture toughness and high strength in an aluminum-lithium alloy.
It has been recognized that the elements lithium, beryllium, boron and magnesium can be added to aluminum alloys to decrease the density. However, current methods of production of aluminum alloys, such as direct chill (DC) continuous and semi-continuous casting, have not satisfactorily produced alloys containing more than about 2.5 wt. % lithium or about 0.2 wt. % boron. Magnesium and beryllium contents up to 5 wt. % have been satisfactorily included in aluminum alloys by DC casting, but the alloy properties have generally not been adequate for widespread use in applications requiring a combination of high strength and low density. More particularly, conventional aluminum alloys have not provided the desirable combinations of low density, high strength and toughness.
The inclusion of the elements lithium and magnesium, singly or in concert, may impart higher strength and lower density to the alloys, but they are not of themselves sufficient to produce ductility and high fracture toughness without other secondary elements. Such secondary elements, such as copper and zinc, often provide improved precipitation hardening response; zirconium may additionally provide grain size control by pinning grain boundaries during thermomechanical processing; and elements such as silicon and transition metal elements can provide improved thermal stability at intermediate temperatures up to about 200.degree. C. However, combining these elements in aluminum alloys forms coarse, complex, intermetallic phases during conventional casting. Such coarse phases ranging from about 1-20 micrometers in size, are detrimental to crack-sensitive mechanical properties, such as fracture toughness and ductility, by encouraging fast crack growth under tensile loading.
Thus, considerable effort has been directed to producing low density aluminum base alloys capable of being formed into structural components. However, conventional alloys and techniques have been unable to provide the desired combination of high strength, toughness and low density. As a result, conventional aluminum based alloys have not been entirely satisfactory for structural applications requiring high strength, good ductility and low density as required in particular applications, including high temperature environments such as internal combustion engines.
A number of aluminum based alloys have been developed in efforts to improve their properties. For instance, U.S. Pat. No. 4,681,736 to Kersker et al discloses an aluminum based alloy which includes 14-18 wt. % silicon, 4-6 wt. % copper, up to 1 wt. % magnesium, 0.4-2 wt. % iron, 4.5-10 wt. % nickel. The aluminum alloy of Kersker supposedly has a fine grain structure, is more castable and its resistance to hot cracking is increased. Moreover, the cast alloy supposedly has a greater ductility.
U.S. Pat. No. 3,765,877 to Sperry et al discloses an aluminum based alloy which includes 7-20 wt. % silicon, 3.5-6 wt. % copper, 0.1-0.6 wt. % magnesium, 1.5 wt. % iron, up to 0.7 wt. % manganese, 2.5 wt. % nickel, 0.5 wt. % zinc, 0.1-1 wt. % silver and 0.01-0.25 wt. % titanium. The aluminum alloy of Sperry et al supposedly demonstrates a high strength and wear resistance.
U.S. Pat. No. 1,799,837 to Archer discloses an aluminum based alloy which includes 7-15 wt. % silicon, 0.3-7 wt. % copper, 0.2-3 wt. % magnesium and 0.4-7 wt. % nickel.
U.S. Pat. No. 4,297,976 to Bruni et al discloses an aluminum alloy which includes 12-20 wt. % silicon, 0.5-5 wt. % copper, 0.2-2 wt. % magnesium, 1-6 wt. % iron, 0.5 wt. % manganese, 0.5-4 wt. % nickel and 0-0.3 wt. % titanium. The aluminum alloy of Bruni et al was particularly developed for piston and cylinder assemblies.
U.S. Pat. No. 4,434,014 to Smith discloses an aluminum based alloy which contains 12-15 wt. % silicon, 1.5-5.5 wt. % copper, 0.1-1 wt. % magnesium, 0.1-1 wt. % iron, 0.01-0.1 wt. % manganese, 1-3 wt. % nickel, 0.01-0.1 wt. % titanium. The aluminum alloys of Smith supposedly demonstrate excellent elevated temperature strength properties and a high modulus of elasticity.
In addition to the above-noted U.S. patents, a number of aluminum based alloys which contain lithium have been developed. U.S. Pat. No. 3,081,534 to Bredzs discloses an aluminum based alloy which contains 1.9-10 wt. % silicon, 0-4 wt. % copper and 0.1-1 wt. % lithium. The aluminum-silicon-lithium alloy of Bredzs was particularly developed as a fluxless brazing or soldering material for aluminum.
U.S. Pat. No. 4,795,502 to Cho discloses an aluminum based alloy which includes up to 5 wt. % silicon, 1.6-2.8 wt. % copper, 1.5-2.5 wt. % lithium, 0.7-2.5 wt. % magnesium and 0.5 wt. % iron. The aluminum based alloy of Cho is prepared by a particular process which supposedly results in an uncrystallized sheet product having improved levels of strength and fracture toughness.
U.S. Pat. No. 4,661,172 to Skinner discloses an aluminum based alloy which includes 0.5-5 wt. % silicon, 0.5-5 wt. % copper, 2.7-5 wt. % lithium, 0.5-8 wt. % magnesium, 0.5-5 wt. % iron, 0.5-5 wt. % manganese, 0.5-5 wt. % nickel and 0.5-5 wt. % titanium. Products from the aluminum based alloy of Skinner are prepared as powder alloys which are rapidly solidified from the melt and then thermomechanically processed into the structure of components supposedly having a combination of high ductility and high tensile strength to density ratios.
U.S. Pat. No. 4,648,913 to Hunt discloses an aluminum based metal alloy which includes 0.5 wt. % silicon, 0-5 wt. % copper, 0.5-4 wt. % lithium, 0-0.5 wt. % magnesium, 0.5 wt. % iron, 0.2 wt. % manganese and 0-7 wt. % zinc. The aluminum based alloy of Hunt is prepared by a process which includes an aging step, and includes a working effect equivalent to stretching in an amount greater than 3% so that after aging, an improved strength and fracture toughness is supposedly imparted to the alloy.
U.S. Pat. No. 4,758,286 to Dubost et al discloses an aluminum based alloy which includes 0.12 wt. % silicon, 0.2-1.6 wt. % copper 1.8-3.5 wt. % lithium, 1.4-6 wt. % magnesium, 0.2 wt. % iron, up to 1 wt. % manganese and up to 0.35 wt. % zinc. The aluminum based alloy of Dubost et al supposedly demonstrates high specific mechanical properties, a low density and good resistance to corrosion.
U.S. Pat. No. 4,526,630 to Field discloses an aluminum based alloy which includes 0.4 wt. % silicon, 0.5-2 wt. % copper, 1-3 wt. % lithium, 0.2-2 wt. % magnesium and 0.4 wt. % iron. The aluminum based alloy of Field supposedly demonstrates improved mechanical properties and the reduction in heat sensitivity.
U.S. Pat. No. 4,735,774 to Narayanan et al discloses an aluminum based alloy which includes 0.12 wt. % silicon, 1.6 wt. % copper, 2.5 wt. % lithium, 1.0 wt. % magnesium 0.15 wt. % iron, 0.05 wt. % manganese and 0.25 wt. % zinc. The aluminum based alloy of Narayanan et al supposedly demonstrates good fracture toughness and relatively high strength.
The present invention is an improvement over the prior art aluminum based alloys and provides an aluminum-lithium alloy having superior characteristics which are ideally suitable for particular applications, including high temperature applications such as mechanical pistons in internal combustion engines.